Biochemically, the cerebellum is characterized by a large concentration of GABA, cGMP, cGMP-dependent protein kinase and a heat-stable protein which inhibits the cGMP-dependent protein kinase. Drug-induced changes in GABA receptor function can be easily monitored by measuring changes in these biochemical parameters. It is suggested that diazepam and muscimol (an indirect and a direct GABA receptor agonist), modulate cerebellar phosphorylation by altering the relationship between cGMP-dependent protein kinase and the content of the cerebellar inhibitor of protein kinase. They could act on the content of the protein kinase inhibitor by decreasing the cerebellar cGMP content with-out modifying that of cAMP.